In the fabricating of electronic devices, the formation of silicon oxide layers to form masks resistant to chemical etches designed for the substrate is known in the art.
Silicon oxide layers have been formed by high temperature thermal oxidation as illustrated in FIG. 1. A device 10 includes silicon substrate 12, a layer 14 of silicon dioxide, which can be grown or deposited by some well-known technique, such as plasma enhanced chemical vapor deposition (CVD). A polysilicon layer 16 can be formed on layer 14 by CVD. Layer 18 is formed of an oxidation mask material, such as a deposited silicon nitride layer. Oxide layer 20 is then deposited on the layer 18. By means of a photolithographic process, portions of the layers 18 and 20 are removed to define an opening, oxygen gas in a high temperature system (e.g. 900.degree. C.) oxidizes the exposed polysilicon 16 to form an oxide mask 22. The oxide mask 22 is self-aligned to the original opening in the pattern defining layers 18 and 20. This feature of self-alignment is often quite advantageous because mask to mask alignment tolerances are increased. However, the process as commonly implemented suffers from several disadvantages. First, a high temperature step (e.g. 900.degree. C. or greater) is required for thermal oxidation and is undesirable in device formation. Second, the grown oxide mask expands laterally so that self-alignment is not precisely achieved. This results in the formation of a "bird's beak", which is shown as number 24. Third, the materal on which the oxide mask is grown is subjected to mechanical stress particularly near the bird's beak regions.
More specifically, the bird's beak is formed during the thermal oxidation of a silicon substrate owing to lateral diffusion of the oxidant species. The term silicon in this specification and claims includes polysilicon. The bird's beak decreases lateral resolution of the device being fabricated.